Method of producing a killed steel

ABSTRACT

A method of producing a fully killed steel of low aluminum content and substantially free of aluminum oxides, without the need for the addition of other metallic or carbidic deoxidizers such as silicon, titanium, silicon carbide, carbon, etc. The steel is produced by the addition of aluminum to molten steel in an amount sufficient to kill the steel and provide a dissolved aluminum content in the order of between 0.03 to 0.06 percent. The steel is then stirred at a controlled rate in contact with an oxidizing slag until the amount of dissolved aluminum remaining in the steel is between 0.005 percent and 0.020 percent. The steel thus produced retains the mode of solidification of a killed steel, is low in dissolved oxygen and aluminum rich oxides, and is especially suitable for continuous casting in that it can be poured through relatively small diameter nozzles without plugging.

W] METHOD or PRODUCING A KILLED STEEL [75] Inventors: Lyall F.Earnhardt, Center Valley;

Bruce C. Whitmore, Bethlehem, both of Pa.

[73] Assignee: John I. Iverson, Bethlehem, Pa.

[22] Filed: May 10, 1971 21 Appl. No.: 141,932

[52] US. Cl 75/129, 75/53, 75/58 [51] Int. Cl. C22c l/06 [58] Field ofSearch 75/58, 129

[56] References Cited UNITED STATES PATENTS 3,634,075 1/1972 Hoff 75/1352,233,726 3/1941 Belding.. 75/58 2,705,673 4/1955 Jordan 75/58 X3,426,833 2/1969 Randak et a1 75/58 X 3,598,383 8/1971 Moore 75/130 X[451 Oct. 30, 1973 Primary ExaminerL. Dewayne Rutledge AssistantExaminerJ. E. Legru Attorney-John l. lverson [57] ABSTRACT A method ofproducing a fully killed steel of low aluminum content and substantiallyfree of aluminum oxides, without the need for the addition of othermetallic or carbidic deoxidizers such as silicon, titanium, siliconcarbide, carbon, etc. The steel is produced by the addition of aluminumto molten steel in an amount sufficient to kill the steel and provide adissolved aluminum content in the order of between 0.03 to 0.06 percent.The steel is then stirred at a controlled rate in contact with anoxidizing slag until the amount of dissolved aluminum remaining in thesteel is between 0.005 percent and 0.020 percent. The steel thusproduced retains the mode of solidification of a killed steel, is low indissolved oxygen and aluminum rich oxides, and is especially suitablefor continuous casting in that it can be poured through relatively smalldiameter nozzles without plugging.

4 Claims, No Drawings 1 METIEOD F PRODUCING, A KILLED STEEL BACKGROUNDOF THE INVENTION tice to reduce the amount of dissolved oxygen in themolten steel by adding metallic or carbidic deoxidizers to the moltensteel. These deoxidizers have a greater affinity for oxygen than doesthe molten steel and will chemically combine with oxygen and remove itfrom solution in the molten steel. One of the most popular deoxidizersused by steelmakers is aluminum, because of its low cost and powerfuleffect as a deoxidizer. I

The oxides resulting from the use of metallic deoxidizers, such asalumina A120 are relatively insoluble in molten steel, and some of theoxide particles remain suspended in the molten steel. When the steel iscast some of these particles become entrapped in the steel as itsolidifies and appear in the finished steel as small non-metallicinclusions. Such inclusions can be a source of surface and internaldefects in the finished steel and can also have a detrimental effect onthe physical and mechanical properties of the steel.

Over the years other deoxidation practices, such as vacuum-carbondeoxidation, have been tried with the aim of minimizing the use ofmetallic deoxidizers to try to avoid introducing non-metallic inclusionsinto the steel. Such practices add to the manufacturing cost of thesteel and frequently have not been fully effective in eliminating theuse of metallic deoxidizers, such as aluminum. Thus, at the present timemany steelmakers prefer to use a powerfuldeoxidizer, such as aluminum,in spite of the danger of introducing nonmetallic inclusions in thefinished steel. 7

The use of aluminum asa deoxidizer has caused a serious problem wheneverthe aluminum killed molten steel is teemed through a relatively smallinternal diameter nozzle, such as in continuous casting. When the steelhas been deoxidized with aluminum, deposits of aluminum oxide form inthe bore and/or ports of the nozzle and disrupt the control of the rateof delivery of the molten steel to the continuous casting machine.

It is therefore an object of this invention to provide a deoxidationpractice for steels which will produce a fully killed steel thatismeasurably lower in oxygen content than conventionally non-aluminumkilled steels.

It is a further object of this invention to provide a deoxidationpractice using aluminum as a deoxidizer for producing fully killed steelthat results in a low aluminum content, a low dissolved oxygen contentand a low content of aluminum-rich oxide inclusions.

It is a further object of this invention to provide a deoxidationpractice for continuous cast steels using aluminum as the deoxidizerwhich will produce a steel that can be teemed through small diameternozzles without the formation of deleterious or restrictive deposits inthe bore or ports of the nozzles.

It is a still further object of this invention to provide a fullykilledsteel which when processed into rolled productswill have excellentsurface and internal quallty.

Other and further objects of the invention will become apparent from thefollowing description and claims.

SUMMARY OF THE INVENTION According to the present invention theforegoing objects are attained by (a) introducing molten steelcontaining dissolved oxygen into a vessel; (b) adding aluminum to thesteel in an amount greater than that requir ed to chemically combinewith allthe oxygen dissolved in the steel and provide a dissolvedaluminum content of between 0.03 and 0.06 percent; (c) providing on thesurface of the molten steel a layer of fluid slag containing iron oxidein an amount greater than that required to chemically combine with theamount of dissolved aluminum retained in the steel; (d) mechanicallystirring the steel at a controlled rate and under conditions to causetransport of the aluminum and aluminum oxides in the steel to theslag-metal interface and cause a reaction of the dissolved aluminum withthe iron oxide in the slag; (e) and then discontinuing the mechanicalstirring when the amount of .dissolved aluminum in the steel is not morethan 0.020 percent and not less than that which would allow thedissolved oxygen concentration to increase to a point where it wouldcombine with the carbon in the steel to form carbon monoxide gas uponsolidificationof the steel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process-according to thisinvention involves initially lowering the dissolved oxygen content of aheat of molten steel by the addition of aluminum as a deoxidizer to thesteel while it is being poured into or in contained in a vessel, such asa ladle. The aluminum is added in an amount greater than that requiredto chemically combine with all of the oxygen dissolved in the steel, sothat after such combinations occur there will remain in the steel in thevessel dissolved aluminum in the order of between 0.03 and 0.06 percent.

The surface of the molten steel in the vessel is covered with a fluidslag layer preferably at least 6 inches thick. This maybe a portion ofthe slag from the steelmaking operation or a specially prepared slaglayer. The slag should contain iron oxide in an amount greater thanrequired to chemically combine with the amount of aluminum dissolved inthe steel. Slags which contain between 12 to 25 percent iron as ironoxide will fit this requirement especially when the slag layer is atleast 6 inches thick.

The molten steel in the vessel is then stirred by any convenient meansat a controlled rate for a predetermined period of time. One low costand convenient method of stirring the molten steel is by bubbling asmall amount of non-reactive gas such as argon, helium or nitrogen, upthrough the steel in the vessel.

The circulation so produced substantially increases the contact betweenthe molten steel and the oxidizing slag layer and promotes a slag-metalreaction that may be characterized by the following formula 2A1 (insteel) 3 P60 (in slag) 3 Fe (in steel) A1203 (solid particles) The A1solid particles so formed are readily absorbed and held by the slag.

in addition, the circulation of the molten steel tends to promote thefloat-out of the aluminum oxide particles that are formed initially inthe molten steel as the product of the deoxidation reaction of thealuminum addition with the dissolved oxygen in the molten steel.

The stirring of the metal can also be produced by other means such as byinduction stirring or with heat resistant paddles, or by continuouscirculation degassers.

The amount and duration of the circulation must be carefully controlled.The stirring is continued until the amount of dissolved aluminum hasbeen reduced to 0.020 percent or less as determined by standardanalytical techniques and terminated before the amount of the dissolvedaluminum approaches zero. It is important that the stirring beterminated when the amount of dissolved aluminum is present in the steelin an amount not more than 0.020 percent and not less than 0.005percent. It has been found that a residual amount of dissolved aluminumin the molten steel in this range will maintain the dissolved oxygencontent in the steel at a value low enough to prohibit a reactionbetween the carbon and oxygen in the steel from occurring onsolidification of the steel.

Under these conditions, we have been able to produce fully killed steelswith total oxygen levels below 50 ppmsand residual dissolved aluminumlevels of less than 0.020 percent, and which have retarded aging ornon-aging properties. Such steels can be continuously cast successfullysince they can easily be poured through the relatively small diameternozzles (less than 1 inch diameter) without plugging the bore or portsof the nozzle or otherwise disrupting the control of the rate at whichthe metal is fed to the continuous casting machine. The reason whykilled steels made in accordance with this invention do not plug thefused silica or other refractory nozzles commercially used in thecontinuous casting of steel is not fully known. However, it has beenobserved that as such steels pass through the nozzle, the rate oferosion of the nozzle bore appears to be approximately the same as therate of accumulation of aluminum oxide in the bore of the nozzle.

During the stirring treatment in the ladle the quantity of aluminum thatcombines with the FeO in the slag is dependent upon the amount ofaluminum in the steel before stirring and the amount of FeO in the slagand the rate and duration of the stirring. The initial aluminum contentin the steel and the FeO content of the slag therefore provides themeans by which one can predict duration of stirring required to producethe desired amount of uncombined aluminum in the molten steel inaccordance with the following formulas:

l. T= Al -0.035/0.006 when slag FeO, is between 15-21 percent 2. T=Al10.035/0.008

grfiater t 2 ..P 2 1.. and where T stirring time 'in minutes using argonas the stirring medium injected at a rate of SCFM and pressure of 80P816. Steel depth in ladle approximately 7 feet; heat size 100 tons;slag thickness approximately 12 inches. Percent A1, analysis of ladlesample for total aluminum before stirring.

when slag FeQ is The slag-metal reaction that is employed in thisinvention may be characterized by the following formula:

2 Al (in steel) 3 FeO (in slag) 3 Fe (in steel) A1 0 (solid particles)The A1 0,. solid particles so formed are readily absorbed and held bythe slag.

The hypothesis described in the technical literature (T. B. King,Electric Furnace Steelmaking, Volume II, Chapter 20, IntersciencePublishers, 1963), as boundary layer theory provides an understanding ofthe mechanism involved in the process.

For two fluids in contact (liquid slag and liquid steel) the theoryinvolves the transport of matter, assuming the existence of boundarylayers adjoining the slagmetal or reaction interface. For the process inwhich we are interested, the pertinent steps involved in the slag-metalreaction may be listed:

1. Transport of aluminum atoms in the metal to the slag-metal phaseboundary.

2. Transport of FeO in the slag to the reaction interface.

3. Reaction between aluminum and FeO at the phase boundary.

In considering the above for definition of the step that controls theprocess rate, the reaction between aluminum and Fe() can be eliminatedsince this is very rapid in comparision to the transport steps. Underthe conditions of stirring, the transport of aluminum in the metal wouldpresent no problem and aluminum concentration at the interface may beconsidered to be essentially the same as that of the bulk metal. Thetransport of FeO in the slag to the slag-metal interface is suggested tobe the rate controlling step for the process.

The rate of the diffusional transport in the slag may be expressed interms of oxygen loss from the slag:

This of course is proportional to the dealuminization rate of the bulkmetal and therefore the rate of aluminum loss from the metal may beexpressed:

where k k D,,"/V

The rate of the dealuminization process in influenced by the imposedagitation since agitation causes the reaction interfacial area (A) tobejncreased and the diffusion boundary layer thickness fiig") to be;diminished. The rate of transport of oxygen to the slag-metal interfaceis influenced by slag FeO concentration (more importantly by FeOactivity in the slag).

The controlled dealuminization reaction occurs without thedeleterioustransfer of oxygen from the slag into the bulk metal due tothe very low solubility of oxygen in iron while finite amounts ofsoluble aluminum are present. Thermodynamic equilibrium solubility ofoxygen in iron at steelmaking temperature (1600 C) is expressed by thereaction: 2 Al (.in steel) 3 0 (in steel) A1 0 (solid) A F =-292,80093.7 T Percent Soluble Equilibrium Percent Oxygen Aluminum Dissolved inFe (Max. Solubility at 1600 C) 0.040 0.00023 0.030 0.0003 0.020 0.000370.010 0.0006 0.005 0.0010

The maximum oxygen solubility values shown on the above table taken fromJ. Chipman and J. F. Elliott, Electric Furnace Steelmaking, Volume II,Chapter 16, lnterscience Publishers, 1963, demonstrate that the changein oxygen dissolved in the bulk metal during and as a result of thedealuminzation treatment is insignificant and beyond the limits ofstandard analytical methods to detect. The key to the process is thediscontinuance of the dealuminization reaction while a finiteconcentration of dissolved aluminum remains in the bulk metal.

As an example of the process according to this invention, a 100 ton heatof steel was made in a basic open hearth furnace and tapped into aladle. Analysis of the steel in the furnace before tapping was Carbon0.05%

Manganese 0.35%

Silicon 0.01%

Oxygen 0.035%

Aluminum 0% Upon tapping, 3.3 pounds of aluminum per ton of steel'wereadded to the steel in the ladle as the steel flowed from the furnaceinto the ladle. The steel in the ladle was at a temperature ofapproximately 2900 F. A one foot thick layer of slag from the furnacewas allowed to accumulate on the surface of the steel. The slag, at tap,had a FeO, content of approximately 16 percent.

Argon gas was injected into the steel near the bottom of the vessel froma hollow refractory covered tube of one inch internal diameter at a flowrate of approximately standard cubic feet per minute and pressure of 80psig. for a period of 3.8 minutes. The steel was then continuously cast.The cast steel had a composition of Carbon 0.05%

' Manganese 0.35%

Silicon 0.01%

vOxygen (Total) 0.004%

Aluminum(dissolved in steel) 0.005%

No problems were encountered with plugging in a submerged-entry tundishnozzle. Port diameters of 56 inch 'in the fused silica tundish nozzlenot only remained free of obstruction from alumina plugging but wereslightly eroded to larger diameter during the casting run. Thecold-rolled sheet processed from the continuous cast slabs had excellentsurface quality, less than 2 percent rejected for surface defectsarising from non-metallic inclusions rich in aluminum oxide.

The example cited above is for a typical l00-ton open hearth heat of lowcarbon killed steel. In the course of our studies, 26 such heats (2600tons) were produced using the practice described. All heats containeddissolved aluminum of 0.005 percent to 0.020 .percent, were completelykilled having total oxygen content of less than 0.005 percent,produced'excellent quality sheet, and were continuously cast by flowingthrough submerged-entry tundish nozzles (port diameter of /2 inch and13/16 inch) without the occurrence of flow restriction due to nozzleplugging.

We claim: 1

1. Method of producing a killed steel especially suit able forcontinuous casting comprising:

a. adding aluminum to molten steel which contains dissolved oxygen in anamount greater than that required to combine chemically with all of theoxygen dissolved in the steel, and produce a dissolved aluminum contentof between 0.03 and 0.06 percent,

prior to, during or after the steel is introduced into a vessel,providing on the surface of said steel a layer of fluid slag containingat least 12 percent iron oxide and in an amount substantially greaterthan that required to combine chemically with the dissolved aluminumretained in the steel,

stirring the molten metal in the vessel to expedite transport ofdissolved aluminum'in the steel to the slag-metal interface and to causereaction of said aluminum with the iron oxide in the slag,

d. discontinuing said circulation when the amount of uncombined aluminumin the steel is not more than 0.020 percent and not less than that whichwould permit reaction between carbon and oxygen in the steel to occur onsolidification of the steel.

2. The method of claim 1 in which the amount of stirring is dependent onthe initial FeO content of the slag.

3. The method of claim lin which the stirring is produced by bubbling anon-reactive gas through the steel.

4. The method of claim 1 in which the fluid slag layer is at least 6inches thick.

. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,Yog 00% Dated CGODGI 30, 1973 Inventor) L all F. Barnlmrdt, et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line 6, "Assignee: John I. Iverson should read Assignee:Bethlehem Steel Corporation Column LL, line 27, "comparision" shouldread comparison Signed and sealed this 30th day of April 19%..

(SEAL) At'test:

EDWARD MJ LETGHER R. 1G. ARSHALL 13mm Attesting Officer Commissioner "ofPatents FORM po'wso (10439) USCOMM-DC c0370 P69 ".5, GOVERNMENT PRINTINGOFFICE: I," 0-36-33

2. The method of claim 1 in which the amount of stirring is dependent onthe initial FeO content of the slag.
 3. The method of claim 1 in whichthe stirring is produced by bubbling a non-reactive gas through thesteel.
 4. The method of claim 1 in which the fluid slag layer is atleast 6 inches thick.